Using the left navigation, ensure you've selected the company you want to add a user to from the Current Company select list.
Again using the left navigation, click on Users and permissions.
Locate the user you want to remove and select Edit.
In the bottom-right corner of the edit user popup, click Remove this user from this company.
In the confirmation popup click Yes, Remove Them. The removed user will no longer be able to access your company projects and orders.